Stabilization of Li Metal Anode in DMSO‐Based Electrolytes via Optimization of Salt–Solvent Coordination for Li–O₂ Batteries

Abstract

traditional Li-ion batteries. [1][2][3] Although extensive worldwide efforts have already been devoted to fabricate an ideal Li-O 2 battery system to realize its practical application, there are still several challenges regarding both air electrodes and electrolytes in rechargeable Li-O 2 batteries, especially electrolyte instability against reduced oxygen species. [4] In a typical nonaqueous Li-O 2 battery, the oxygen reduction process results in the formation of the superoxide radical anion (O 2 •− ), lithium superoxide (LiO 2 ), lithium peroxide (Li 2 O 2 ), and others. The O 2 •− , LiO 2 , and Li 2 O 2 have very high reactivity towards most of the components inside the Li-O 2 battery such as carbon-based air electrodes, polymer binders, and organic electrolytes, [5,6] especially the nonaqueous electrolytes, [7][8][9][10][11][12][13][14][15][16] including solvents [6,17] (such as the organic carbonate solvents widely used in Li-ion batteries, [9,12] ether-based solvents, [13,14] and ionic liquids [15,16] ) and salt anions. [10] It is known that most organic solvent systems are not stable against reduced oxygen species, especially O 2 •− . Although some ether-based solvents are relatively stable and have been widely used in Li-O 2 battery studies, further improvement of their stability during Li-O 2 reactions still remains a major challenge for Li-O 2 batteries. It is worth noting that using superconcentrated electrolytes (including ether-based, [18] dimethyl sulfoxide (DMSO)-based, [19] or acetonitrile-based [20] ) can enable enhancement of electrolyte stability for fast-charging Li-ion batteries, which presents a promising superconcentrated-salt strategy. Besides the use of aforementioned electrolytes in Li-ion batteries, use of highly concentrated electrolytes (i.e., lithium bis(trifluoromethanesulfonyl) imide (LiTFSI)-anhydrous 1,2-dimethoxyethane (DME), [13] LiTFSI-tetraethylene glycol dimethyl ether (TEGDME), [21] and lithium bis(fluorosulfonyl)imide (LiFSI)-DME [22] ) also brought new insights into electrolyte selection in Li-O 2 and Li metal batteries.Recently, the organic solvent DMSO has been used in Li-O 2 batteries to enhance the performance of Li-O 2 batteries due to the relative stability of DMSO against LiO 2 as well as its enhanced oxygen diffusion coefficient and high Li + conductivity compared to other solvents. [23][24][25][26][27][28] Bruce and co-workersThe conventional electrolyte of 1 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethyl sulfoxide (DMSO) is unstable against the Li metal anode and therefore cannot be used directly in practical Li-O 2 batteries. Here, we demonstrate that a highly concentrated electrolyte based on LiTFSI in DMSO (with a molar ratio of 1:3) can greatly improve the stability of the Li metal anode against DMSO and significantly improve the cycling stability of Li-O 2 batteries. This highly concentrated electrolyte contains no free DMSO solvent molecules, but only complexes of (TFSI − ) a Li +  (DMSO) b (where a + b = 4), and thus enhances their st...

Alkali‐Oxygen Batteries Based on Reversible Superoxide Chemistry

“…An important characteristic for the electrolyte is its ability to dissolve the superoxide product. It has been shown that high‐donor solvents, such as DMSO, increase Li + solvation and increase solubility of LiO 2 . This is important for maximizing discharge capacity because it facilitates crystal formation, which is a solution‐mediated process …”

“…It has been shown that high-donor solvents, such as DMSO, increase Li + solvation andi ncrease solubility of LiO 2 . [54,55] This is important for maximizingd ischarge capacity because it facilitates crystal formation, which is as olutionmediated process. [39] Directly relatedt ot he stabilityo ft he anode, is the stability of the electrolyte during operation of the superoxide battery.…”

“…This is essential,a si ti sf or any battery system.I ns ome ways, the same considerations for Li-O 2 batteries apply to superoxide-based batteries, since the superoxidea nion is generated in both cases. The use of highly concentrated electrolytes has been shown to provide two main advantages relevant to metal-oxygen batteriesw ith both DME-and DMSO-based electrolytesb yL iu et al [56,55] The first is that the SEI layer formed on the metal anode is both more compact( i.e.,l ower impedance) and possesses greater stability in the presence of O 2 crossover. Second, concentrated electrolytes also impart an increasedo xidative stability of the electrolyte at the cathode.…”

Review of Electrolytes in Nonaqueous Lithium–Oxygen Batteries

“…Therefore, Zhang and co-workers demonstrated that DMSO electrolyte could significantly enhance the stability of lithium metal and the cycling performance of the Li-O 2 batteries through adjusting the concentration of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in DMSO electrolyte (Figure 9). [18] The highly concentrated DMSO electrolyte contains only complexes of (TFSI − ) n -Li + -(DMSO) 4−n instead of free DMSO, thus increasing its stability with lithium metal anodes. Moreover, these complexes have higher Gibbs activation energy barriers than the free DMSO, indicating enhanced stability of the electrolyte against attack by superoxide radical anions.…”

“…[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Several excellent reviews have been published to summarize recent developments and our current understanding of Li-O 2 batteries, especially with respect to the cathode electrocatalytic materials. [25][26][27][28][29][30][31][32][33][34][35] These reviews have comprehensively discussed the relationship between the structure of electrocatalytic materials and the electrochemical performance of Li-O 2 batteries.…”